What will happen to my FH if I overfeed it? Usually if I feed them shrimp they can eat it like A LOT. What will happen if I overfeed it? Thanks in advance 
You mean the growth of the belly or just temporary?It will get huge belly and over feeding will make your tank dirty
What's HITH?This is the fish in a 15 gallon tank with a pleco, so overfeeding is a bad idea. Also, the pleco is a dirty fish to begin with and in a small area waste food from overfeeding the FH will cause more problems. The extra fecal matter from overfeeding will make matters worse. I hope you are doing at least two water changes per week.
My prediction is that your FH comes down with HITH before it reaches a length of 3.5 inches if you do not address these areas of concern.
Okay bro, thanks for the advice. Appreciate itHITH = Hole In The Head. Do some research on Heximita. It's just one a many ailments that cramped, stressed fish in dirty water will suffer causing a decrease in their lifespan. Due to inbreeding, Flowerhorns already have a shorter lifespan than the CA/SA cichlids that they are hybridized from, so keeping a FH in less than ideal conditions doesn't do the fish any favors.
Well I removed almost all the coral inside and left gravels for my FH to playLeviathan in order to asure your fish well being in that reduced set up, you could take out the whole gravel and leave it bare bottom, so your filter (assuming youre using a cascade or a canister filter) could retire all the waste.
You can use some mangler roots and one clay pot for your fish to hide and that's it, with these measures and at least 25%-30% weekly water changes you wouldn't have any problem at all in the meantime you upgrade.
Cheers.